694 NE^^' york state museum 



Isograptus gibberuliis Moberg. Greol. Foren. Stockb. Forh. 1892. 14:346, 



pl.8, fig.3-7 

 Didymograptus gibberulus Holm. Sver. (T-eol. Und. Afh. oeb upps. 1895. 



ser.C, no.l50, p.lS 

 Didymograptus (Isograptus) gibberulus Romer & Freeh. Lethaea 



palaeozoica. 1897. 1:593, lig.161 

 Didymograptus gibberulus Elles. Quar. Jour. GeoL Soc. 1898. 54:496 

 Isograptus gibberulus Tornquist. Lunds Univ. Arsskrift. 1901. Bd37, 



Afd.2, p.23, pl.3, %.16-19 

 Didymograptus gibberulus Elles & Wood. Monogr. Brit. Grapt. ptl. 



Pal. Soc. vol. for 1901. p.52 ; p.53, fig.33a and b, pl.2, %.9a-9e 



Description. Primary disk not observed. Nema filamentous, very thin 

 and long. Sicula long and slender (lengtli about 3 . 2 mm). Two branches 

 wMcli, bending in opposite directions upward and slightly backward, form a 

 rhabdosome of horseshoe shape ; the angle of divergence of the branches vary- 

 ing, in the material, between 800° and 830°. The branches are short (greatest 

 length observed 5 . 6 mm) ; widest at their base (about 2 . 2 mm) and diminish- 

 ing in width toward the distal ends to one half their original width, this 

 diminution being produced by a change in the direction of the thecae from 

 straight downward to obliquely outward and upward. Thecae long (the 

 proximal ones about 2 mm long) numbering 11 to 14 in 10 mm, slightly curv- 

 ing with the convex side upward, inclined in their distal part toward the axis 

 at an angle of 45°, three times as long as wide, in contact throughout. Aper- 

 tural margin concave, mucronate at the lower end and receding upward. 



Position and localities. At the Deep kill this form has been obtained 

 only in half a dozen specimens found on a slab which was picked up loose 

 at the outcrop of graptolite horizon 2 (zone with D. bifid us). I have 

 not been able to locate the bedding surface which is covered with this inter- 

 esting form, but, from the lithologic aspect of the slab, I believe that it came 

 from the eastern part of the quarry (graptolite beds 5), which in some parts 

 is little accessible now. Numerous specimens of a much dwarfed mutation of 

 this species [cited 1902, p.570, as D. (Isograptus) gibberulus 

 Nich. var. nanus var. nov.] were found in graptolite bed 7 (horizon with 



